The expression allows you to animate the Opacity parameter so that after the marker the transparency gradually decreases to zero, and on the next marker it gradually increases to 100%. Moving the markers, the transparency animation will move along with them. Thus, you can arrange any number of markers and animate fades without using keys. To put a marker on the layer, you must press the [*] button.

if (marker.numKeys > 0){ m = marker.nearestKey(time);
frIn = 5; // Duration of appearance (in frames)
frOut = 15; // Duration of disappearance (in frames)
t = m.time;
if (m.index%2){ ease(time,t,t+frOut*thisComp.frameDuration,100,0) }else{ ease(time,t,t+frIn*thisComp.frameDuration,0,100) } }else{ value }